1. Field of the Invention
The present invention relates to fabrication of a structure (such as optical transceiver) in which both of rigid boards mounted on, for instance, different objects are connected with each other by a flexible board, and more particularly, to a board assembly, an optical transceiver using the same and a method for mounting the same on the objects.
2. Description of the Related Art
An optical transceiver is used as an interface for connecting an optical transmission path and an electric communication device, which conducts an electrical processing of optical transmission signal of the optical transmission path. For achieving this purposes, the optical transceiver is provided with a built-in optical module and built-in electric circuit board(s). The optical module is an integrated module comprising an optical fiber coupled to the optical transmission path, a lens for focusing a light transmitted through the optical fiber, and an optical element for receiving the focused light (or for transmitting the light, adversely). A lead of the optical module for leading an electric current from the optical element is usually provided to project from an end of the optical module, and another end of the optical module is provided to face to the optical transmission path. On the other hand, the electric circuit board is provided in vicinity of the lead so as to shorten a distance from the optical element.
For connecting a number of optical transmission paths to a single communication device, components of the optical transceiver are required to be collectively disposed. For answering to this request, the optical transceiver is provided with a housing, which accommodates the electric circuit board in a longitudinal direction of the optical module. The vertical length and lateral width of the housing are short, and only a length in a depth direction of the housing is long. Accordingly, the electric circuit board accommodated in this housing naturally becomes elongated. When the lead projected from the optical module is directly fixed by solder to an end of the electric circuit board, stress is easily applied to this soldered part, lead or optical element, as well as an optical axis of the optical module is easily deviated.
Therefore, as shown in FIG. 1, a device (called as “board assembly”) has been conventionally used. In the board assembly, a first rigid board 53 to which terminals 55 of an optical module 51 are soldered and a second rigid board 54 on which an electric circuit (not shown) is mounted are formed separately, and both of the first and second rigid boards 53, 54 are connected with each other by a flexible board 52. Japanese Patent Laid-Open (Kokai) No. 8-136765 (JP-A-8-136765) discloses such board assembly. A part of the electric circuit can be mounted also on the first rigid board 53. An optical module stay member (not shown) for accommodating the optical module 51 and a rib (not shown) for fixing the second rigid board 54, on which the electric circuit is mounted, are formed in a housing (not shown). After putting the optical module 51 in the optical module stay member, the board assembly is carried into the housing. Then, the second rigid board 54, on which the electric circuit is mounted, is fixed by screws to the housing that is the object, while the first rigid board 53 is fixed to the optical module 51 which is the object by soldering the terminals 55. Finally, the housing is closed by a cap (not shown), such that the optical module 51 is pressed into the optical module stay member.
As described above, by providing the board assembly in which both of the first and second rigid boards 53, 54 are connected by the flexible board 52, even if different forces function to the first and second rigid boards 53, 54, or if the first or second rigid board 53, 54 is out of position, a stress will be absorbed by transformation of the flexible board 52. Accordingly, strain of the leads or misalignment of an optical axis of the optical module 51 can be relaxed.
In the prior art shown in FIG. 1, the second rigid board 54 is mounted on the housing that is the object in a substantially parallel position to the optical axis of the optical module 51, and the first rigid board 53 is mounted on the optical module 51 that is the object in a position orthogonal to the optical axis of the optical module 51, so that the flexible board 52 is bent with a predetermined curvature (radius) R at an area around the intersection of extended planes of the first and second rigid board 53, 55. When observed from a larger viewpoint, the flexible board 52 is bent with a right angle.
However, since the first and second rigid boards 53, 54 are mounted on the each object in different positions, the stress is applied to the flexible board 52. The flexible board 52 has elasticity, so that a repulsive force functions to turn back a curved portion to a state of an original flat plane. Since the optical module 51 receives this repulsive force, a risk of causing the strain of the leads or the misalignment of the optical axis of the optical module cannot be completely overcome.
In addition, it is a complicated operation to apply the bending force to the flexible board 52 while carrying the board assembly into the housing, and it is inevitable to apply unnecessary force to the optical module 51, or to each of the first and second rigid boards 53, 54 during this process.
Further, a certain kind of the flexible board comprises a first conductive layer referred as a “signal layer” on which signal wiring pattern is provided, and a second conductive layer referred as a “ground layer” provided separately from the first conductive layer. Namely, in this kind of flexible board, a ground layer 62 and signal layer 64 are interposed by protective layers (insulating films) 61, 63, and 65 as shown in FIG. 2. The ground layer 62 is a so-called mat (plane) ground layer, which occupies an entire surface or almost entire surface of the flexible board, and has an effect of lowering ground impedance and increasing a shielding property, etc. However, since one more conductive layer is provided, hardness of the flexible board is increased, so that the repulsive force to turn back the curved portion to the original flat plane is increased. Therefore, the aforementioned problems of the strain of the leads or misalignment of the optical axis become more serious.